Flexography is a method that is commonly employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils, and laminates. Newspapers and paper cups are prominent examples. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Photosensitive elements generally have a layer or layers of a photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, or photocuring of the photopolymerizable layer occurs in the exposed areas. It is then required that these plates be developed, and it is commonly known to the art to treat them with a solvent or aqueous based washout to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. Alternatively, it is known to the art to utilize a “dry” thermal development, in which the differences in melting temperature between the polymerized and unpolymerized portions of the plate are exploited to leave behind the appropriate relief structure.
Many flexographic plate producers use high pressure water spray to process their printing plates. Examples of these systems are described in U.S. Pat. No. 4,196,018 to Inoko et al. and U.S. Pat. No. 4,081,815 to Horner. These processes, while effective in removing the uncured photopolymer, have a number of deficiencies. First, this approach requires plumbing to transport the water to the process and energy to heat this water. Also, the process generates aqueous effluent, which must then be disposed. Often, treatment is needed before the effluent is disposed of, adding to the expense. However, most importantly for the present invention, the high pressure water spray is ineffective in creating a change in the surface of the cured photopolymer. Importantly, the producers of these flexographic plates are unable to accurately regulate the surface roughness of the relief structure, whether to make it substantially smooth or to have a uniform surface roughness.
“Dry” thermal development is a process that has been gaining popularity for the production of flexographic plates. It is a user friendly process that produces a high quality plate. The basic parameters of the process are well known to the art, as described in U.S. Pat. No. 5,175,072 to Martens. These processes allow for the elimination of development solvents and the lengthy drying times needed to remove the solvent. Dry blotting is used to remove the uncured and melted photopolymer, leaving behind the cured photopolymer with the higher melting temperature so that a relief structure is formed. However, just as with the solvent method of development, the producers of these flexographic plates are unable to effectively regulate the surface roughness of the relief structure. The blotting rollers only function to remove the uncured photopolymer, and for a variety of reasons cannot reliably change the surface structure.
U.S. Pat. No. 5,279,697 to Peterson et al. and U.S. Pat. No. 7,241,124 to Roberts et al. depict typical thermal development devices using a hot roller and an absorbent material that is contacted with the heated printing plate. The absorbent material consists of a heated webbing that allows for the softened or liquefied photopolymer to be absorbed into the web.
The present invention provides a novel system for the thermal based development of flexographic printing plates that overcomes many of the disadvantages of the prior art.
The present invention is directed to a novel process and apparatus for thermally developing flexographic printing plates comprising the use of a roller or other means that substantially changes the surface formation of the cured photopolymer to become either smoothed or roughened and preferably uniformly smoothed or uniformly roughened. This roller can be placed either before or after (preferably after) the previously mentioned blotting rollers and can be either coated with a resilient surface such as TEFLON® or rubber or uncoated. Unlike thermal development systems of the prior art, the system of the present invention achieves a substantially uniform surface formation in the relief structure that aids in uniform printing.
The present invention also provides a novel system for the solvent based development of flexographic printing plates that overcomes many disadvantages of the prior art. The spraying of water or other solvents on the cured and uncured photosensitive elements does not reliably effect a change in the surface formation of the relief structure. As with thermal development, solvent based development is only aimed at removing the uncured photopolymer, and does not effect a substantial smoothing or uniform roughening in the relief structure of the plate.
Furthermore, customers frequently demand that flexographic printing elements be suitable both for thermal development and for development by means of solvents. However, the surfaces of the relief structures formed by these two methods are not necessarily similar, and these differences would lead to inconsistent coatings, especially if both thermal and solvent developed printing plates are used to print the same patterns. The addition of a roller that does not comprise a blotting system, but which aims at improving the surface formation of the relief structure, overcomes disadvantages in the prior art. Using this, the relief structures of the printing plates can be smoothed so that when the image is printed, it is uniform regardless of the type of development it went through.